KasiCair
I just thought of this author! Has rei and his girls soul bonded with Kaguya yet, or is the corrupted chakra a problem? If Kaguya joins then all of them will have what can only be described as world-destroying powers! Kaguya will also get better chakra control to massively boost her as you talked about before, seal prowess that might make her like seals instead of tolerating them, and nature energy talent that might let her do things about her corruption. That last one could be good! She can either manually suppress it, separate it, or use it as a weapon to corrupt minds! She'll need tons of training of course!